For improved television transmission systems it is preferable, particularly with regard to acceptance by the public, to increase the picture aspect ratio of television receivers from the preexisting standard aspect ratio of 4:3 to 16:9, which is the aspect ratio recommended for future HDTV standards, without losing compatibility with existing television standards such as PAL, NTSC and SECAM.
Various proposals have been made for achieving such a wider aspect ratio. Some of these proposals are illustrated by FIGS. 1(a) through 1(i).
The proposal according to FIG. 1(a) illustrates the scanning of wide screen motion pictures. A picture section with a 16:9 aspect ratio is generated by omitting lines at the top and bottom picture edges of a 4:3 aspect ratio picture. For example, with PAL and SECAM standards this means a line reduction from 575 active lines to 431 active lines. A compatible 4:3 receiver (FIG. 1b) would show the complete picture, but with only 431 active lines as well as black bars along the upper and lower picture edges. These black bars would not be present in a 16:9 receiver (FIG. 1c), but the resolution would then be reduced due to the reduced number of lines compared to a standard scanning format with 575 active lines.
In the proposal according to FIG. 1 (d), the line length is increased in relation to the picture height during the scanning process at the picture source, so that a picture with a 16:9 aspect ratio results. The side panels which complement the 4:3 aspect ratio to form a 16:9 aspect ratio image are transmitted in such a way that they cannot be processed by a standard 4:3 receiver (FIG. 1e). This results in a reproduced image without reduced vertical resolution (for example, 575 active lines with the PAL or SECAM standard). However, the picture information contained in the side panels is lost in the 4:3 receiver. The 16:9 receiver (FIG. 1f), on the other hand, reproduces the complete picture information with 575 active lines. However, the picture information contained in the side panels (FIG. 1d), which is transmitted and received separately, is spliced to the main panel, leading to visible seams.
In the proposal according to FIG. 1(g) (for example, according to D2-MAC specifications) the line length is also increased in relation to the picture height during the scanning process at the picture source, so that again a picture with a 16:9 aspect ratio is generated. Compared to the preceding proposal, however, the side panels are transmitted continuously rather than separately. The 16:9 receiver (FIG. 1i) therefore reproduces a picture with full picture information and 575 active lines, without the disadvantage of seams at the side panel splicing regions. Instead, geometric distortion occurs in a standard 4:3 receiver (FIG. 1h) if no additional measures are provided. With MAC receivers it is envisaged to modify the vertical amplitude or the time decompression process by means of a transmitted control signal which compensates for geometric distortion. However, picture information is lost at a 4:3 receiver if the line compression is altered. In the case of a change of the vertical amplitude as represented in FIG. 1(h) black bars appear along the top and bottom picture edges, however, complete picture information is reproduced. With conventional television standards such as PAL, NTSC and SECAM, however, automatic geometry correction is not impossible.
It is the object of the invention to provide a television transmission system compatible with conventional television standards (PAL, NTSC, SECAM) which allows, when a 16:9 aspect ratio picture source is used, reproduction by a 16:9 receiver with substantially full image quality, and without producing picture distortion in the case of standard 4:3 television receivers.